AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams contains many details to which we should pay close attention when performing necessary measurements for the calibration of high-energy beams. One such measurement is calculating the polarity correction factor (Ppol) for ionization chambers used in electron beam dosimetry. Since polarity effects vary with beam quality and other conditions (such as cable position), one must correct for these effects by making measurements each time clinical reference dosimetry is performed. To correct an ion chamber’s raw reading for polarity effects it is necessary to take readings with both polarities applied and tabulate Ppol by the following equation: Read more

5- The PTV used for brachytherapy planning is typically smaller than the PTV for external beam technique, however, brachytherapy results in a significant dose inhomogenity.

Don’t take the exam if you don’t know the answer of this question.

Some of our readers have recommended that we do not post the answer at the same time as we post the question. We think it is a good suggestion, as it gives the readers a chance to think about the answer. Therefore, the answer will be posted within a couple of days.

Note: Co-60 machines may be out of the picture, but there may be a question on C0-60 machines on the exam. Although Time Error can be applicable to all therapy machines, it’s more applicable to Co machines.

In a shielding evaluation (Survey) of a new vault that housed a Varian linear accelerator, In-Any-One-Hour Time Averaged Dose Equivalent Rate (Rh) was found to be 2.5 mr/hr in the unrestricted area. Which of the following are true? (There may be multiple answers)

1- Rh is only applicable for a restricted area.

2- It is over the limit for restricted area and some action should be taken.

3- It is over the limit for unrestricted area and some action should be taken.

4- Rh is only applicable for radioactive materials shielding survey.

5- Value of Rh depends on average number of patient treatments in any one hour and the weekly time averaged dose equivalent rate.

For those of you preparing to take the ABR exam this September, here is a problem for you (because it’s August–one month away from your test date–and you can never do too many practice problems!). Stay tuned for more practice problems in the coming month.

A prostate I-125 seed implant patient would like to know what fraction of the dose is delivered to the tumor in 10, 60, and 80 days after the implant.